It is known in the art to combine polypropylene with an olefin copolymer such as an ethylene-propylene copolymer and an ethylene-1-butene copolymer, in order to improve impact resistance of the polypropylene. As a process for producing such an olefin copolymer, EP 264169B discloses a process for producing an ethylene-propylene copolymer rubber, comprising copolymerizing ethylene and propylene in the presence of a polymerization catalyst composed of a catalyst component and an organoaluminum compound, the catalyst component being obtained by contacting (A) a metal oxide, (B) a dihydrocarbyl magnesium, and (C) a hydrocarbyloxy group-containing compound with one another to form a first contact product, contacting the first contact product with (D) a halogen-containing alcohol to form a second contact product, and finally contacting the second contact product with (E) a titanium compound to form a titanium-containing catalyst component.